Modern mobile wireless power amplifiers are typically constructed as an integrated circuit, employing a variety of active transistors and passive components. Presently, the prevalent technologies for such integrated circuits are based on heterojunction bipolar transistors (HBT) devices, especially those derived from GaAs, AlAs, InAs, InP, or various alloys thereof. Such circuits are often utilized as a component in a larger integrated product and may include RF switching and filtering functions and matching networks. These integrated products are designed to amplify an input low power RF signal, and output a high power modulated signal that is suitable for wireless transmission over distance.
Heat dissipation is becoming an increasingly important issue for mobile power amplifiers. Especially of concern in this area are the transistors, generally HBTs, upon which the amplifier integrated circuit is based. Notably, product reliability may be limited not only by the maximum junction temperature for the transistors, but also by local thermal nonuniformities along the transistor dimensions. In the first case, operation of the circuit at a junction temperature in excess of the device rating leads to reliability failure in the transistor junctions, contacts, and metallizations. In the second case, operation of the transistor in a condition of thermal nonuniformity may lead to localized reduction of the transistor turnon voltage, which in turn may lead to a runaway condition in which the transistor catastrophically fails at the hot spot. Further exacerbating the requirements for thermal dissipation and uniformity, recently the industry trend is to reduce the power amplifier die and package sizes. Smaller power amplifier die and amplifier packages are desirable for both cost and form factor reasons. For example, in thin and compact modern wireless handsets, smaller form factors are desired. However, the decreasing die and package size increases the thermal power density on the die and the package, and may also increase the local thermal nonuniformity of the transistors. These factors further increase the need to dissipate the heat generated at the device level.
What is needed, therefore, is a semiconductor structure that overcomes at least the shortcomings described above.